The Subdivision Modifier Keeps Causing My Models To Warp Inside Of Itself
The topology of your model is not suitable for subdivision surface. Subsurf works best with quads, and the entire side of your gun model is made up of ngons.

If you are looking for a quick way to chamfer the edge of the model, consider using a bevel modifier, which works nicely with ngons.



(Excuse the ridiculous model, hacked it together on a laptop in 30 seconds.)